1. Field of the Invention
This invention relates to machine vision systems and symbology readers that employ machine vision and more particularly to illuminators for the same.
2. Background Information
Machine vision systems use image acquisition devices that include camera sensors to deliver information on a viewed subject. The system then interprets this information according to a variety of algorithms to perform a programmed decision-making and/or identification function. For an image to be most-effectively acquired by a sensor in the visible, and near-visible light range, the subject should be properly illuminated.
In the example of symbology reading (also commonly termed “barcode” scanning) using an image sensor, proper illumination is highly desirable. Symbology reading entails the aiming of an image acquisition sensor (CMOS camera, CCD, etc.) at a location on an object that contains a symbol (a “barcode”), and acquiring an image of that symbol. The symbol contains a set of predetermined patterns that represent an ordered group of characters or shapes from which an attached data processor (for example, a microcomputer) can derive useful information about the object (e.g. its serial number, type, model, price, etc.). Symbols/barcodes are available in a variety of shapes and sizes. Two of the most commonly employed symbol types used in marking and identifying objects are the so-called one-dimensional barcode, consisting of a line of vertical stripes of varying width and spacing, and the so-called two-dimensional barcode consisting of a two-dimensional array of dots or rectangles.
By way of background FIG. 1 shows an exemplary scanning system 100 adapted for handheld operation. An exemplary handheld scanning appliance or handpiece 102 is provided. It includes a grip section 104 and a body section 106. An image formation system 151, shown in phantom, can be controlled and can direct image data to an onboard embedded processor 109. This processor can include a scanning software application 113 by which lighting is controlled, images are acquired and image data is interpreted into usable information (for example, alphanumeric strings derived from the symbols (such as the depicted two-dimensional barcode image 195). The decoded information can be directed via a cable 111 to a PC or other data storage device 112 having (for example) a display 114, keyboard 116 and mouse 118, where it can be stored and further manipulated using an appropriate application 121. Alternatively, the cable 111 can be directly connected to an interface in the scanning appliance and an appropriate interface in the computer 112. In this case the computer-based application 121 performs various image interpretation/decoding and lighting control functions as needed. The precise arrangement of the handheld scanning appliance with respect to an embedded processor, computer or other processor is highly variable. For example, a wireless interconnect can be provided in which no cable 111 is present. Likewise, the depicted microcomputer can be substituted with another processing device, including an onboard processor or a miniaturized processing unit such as a personal digital assistant or other small-scale computing device.
The scanning application 113 can be adapted to respond to inputs from the scanning appliance 102. For example, when the operator toggles a trigger 122 on the hand held scanning appliance 102, an internal camera image sensor (that is part of the image formation system 151) acquires an image of a region of interest 131 on an object 105. The exemplary region of interest includes a two-dimensional symbol 195 that can be used to identify the object 105. Identification and other processing functions are carried out by the scanning application 113, based upon image data transmitted from the hand held scanning appliance 102 to the processor 109. A visual indicator 141 can be illuminated by signals from the processor 109 to indicate a successful read and decode of the symbol 195.
In reading symbology or other subjects of interest, the type of illumination employed is of concern. Where symbology and/or other viewed subjects are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, light that strikes the subject nearly perpendicularly (normal) or at an angle that is typically no more than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, barcodes and other subjects requiring mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of item or container.
Conversely, where a symbology or other subject is formed on a more-irregular surface, or is created by etching or peening a pattern directly on the surface, the use of highly reflective bright field illumination may be inappropriate. A peened/etched surface has two-dimensional properties that tend to scatter bright field illumination, thereby obscuring the acquired image. Where a viewed subject has such decidedly two-dimensional surface texture, it may be best illuminated with dark field illumination. This is an illumination with a characteristic low angle (approximately 45 degrees or less, for example) with respect to the surface of the subject (i.e. an angle of more than approximately 45 degrees with respect to normal). Using such low-angle, dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for better image acquisition.
Often, an area of interest is best illuminated with highly diffuse illumination, typically projected at a relatively high angle onto the surface being viewed. Commonly owned U.S. patent application Ser. No. 11/257,411 entitled INTEGRATED ILLUMINATION ASSEMBLY FOR SYMBOLOGY READER, by Laurens W. Nunnink, et al., the teachings of which are expressly incorporated herein by reference, describes a handheld symbology reader with an illumination assembly that incorporates a light pipe projecting a dark field illumination pattern surrounding a conical diffuser that, via a reflector transmits a direct diffuse illumination pattern from a plurality or rearward-projecting light emitting diodes (LEDs). This diffuser employs a multi-step reflector to better spread the light from the rearward projecting LEDs throughout the conical surface for the best spread pattern.
While a diffuser of this type is highly effective in illuminating surfaces, it requires a central aperture through which the imager lens receives light from the subject. This aperture, along with the need to redirect and spread the light from a limited number of discrete number of light sources into a wide field, may result in some regions on the surface that are more-dimly lit than others (a mild spotting effect).
Conversely, a diffuse illuminator employing a perimeter array of light sources that project light inwardly to a regularly spaced grid assembly is commercially available from CCS of Japan. The grid assembly is, in essence a plate of transparent material having a series of reflective spots (a “grid”) applied to one side and opposite a fully transparent side. The light is internally reflected to bounce off of the spots and through the opposing transparent side, and toward a subject. The image resides on the grid side of the illuminator and views the subject through the grid. This illuminator provides a very consistent light across its surface that is substantially unbroken due to the lack of a large central aperture and that avoids spotting effects due to the even spread of light around the matrix of the grid. This light spread is generated by a substantially continuous array of light sources (red LEDs, for example) that are provided around the perimeter of the grid. In particular, the unique geometry of the grid projects the array's light downwardly onto the subject, while blocking upward transmission of light from the array. The grid includes sufficient open space between grid elements (reflectors) so that, at the camera's normal standoff from the grid, it appears largely invisible in the acquired image, much like a window screen viewed at a distance.
However, this type of diffuse illuminator is specifically adapted for fixed-mount applications, in which the subject is provided at a stationary inspection location with a fixed camera located thereabove. The illuminator is also relatively large in footprint (30 or more centimeters square in many versions. In addition, the illumination array is housed in a wide framework that encircles the grid, thus providing a significant degree of non-projecting space along the overall surface area of the grid. In all, this type of illuminator is not adapted for use in a movable handheld environment where the maximum amount of surface area should be devoted to illumination and the overall surface area should be minimized.